5 Answers
5

It depends. It depends mainly on how users will be locating the data they are interested in.

Numerical Stats in a Row

If the page is repeating the same stats groupings in the same order, then positional memory will be used, and the numbers themselves also cue the reader in to positioning - Best bowling 5/45 has a different form to Economy Rate 1.51 and people will use that cue. When positional memory is being used bold numbers and quiet labels are good. The user is, for example, comparing bowling stats of multiple players.

Header to a Report:

In a written report, and by extension the same in web format, it is usually better to bold the labels in a header:

Project: Project 123Phase: Phase ABCDepartment: Department XYZ

Positional memory within the group is less relied on. The typical activity is not scanning multiple reports to find what phase each one relates to or compare project titles, rather it's having read one report, checking what phase that one report relates to or exactly what the project title was. The scanning is interspersed with a different activity of long duration. Finding the header information at all is the main scanning.

Also a factor for bold labels - Longer lists defeat positional cuing more, and bias towards embolding the labels. So a rule of thumb:

Long lists (>5 items) seen rarely (<10 a month): Bold the label.
Short lists (<5 items) seen frequently (>10 a month): Bold the information

If this is a dashboard like situation where a visual hierarchy needs to be formed, then my first guess would be to bold the data. Like jonshariat mentioned, this will make it easier for users to scan the page and find the data they are looking for. On the other hand, if this is in a table or in an area where there is a ton of data present, you may want to bold the label because bolding the data will do no good if everything is bold.

Best way is to design both, show them to people, get feedback. Then use your judgement as to which direction you want to go.

For horizontal statistics, I find it better to bold the label, so that the user can easily find the statistic they are looking for, but for vertically centered like those pictured, it could be done either or.
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Nick BedfordJun 7 '11 at 1:14

Where the point of the data is to allow a comparison (Which project is the furtherest along, and by how much?), bolding the relevant data will let a user focus on that more easily.

Similarly, when the point of the data is to allow a lookup of information (which department do Rachel, John, and Steve work in?), bolding the relevant labels will allow a user to narrow down on the data they're searching for.